At what speed do all wizards apparate? Is it controlled at the will of each individual wizard or it is constant for all?
What's the speed (or, upper speed limit)? Is it sub-light speed or light speed? Or, even faster than light (I don't think Einstein stands in front of magic)?
I'm quite sure Apparition is much slower than light speed. There's a passage in Price that seems to indicate that at least a few tenth of a second but possibly seconds pass during an Apparition. This is the wizard's subjective time, but I think it's roughly the same as the externally observed time.
Quoting Half-Blood Prince chapter 4, when Harry Apparates side-along with Dumbledore.
Harry felt Dumbledore's arm twist away from him and redoubled his grip: the next thing he knew, everything went black; he was being pressed very hard from all directions; he could not breathe, there were iron bands tightening around his chest; his eyeballs were being forced back into his head; his eardrums were being pushed deeper into his skull, and then–
He gulped great lungfuls of cold night air and opened his streaming eyes. He felt as though he had just been forced through a very tight rubber tube. […]
(Portkeys appear to work the same, because the books talk about similar sensations when wizards travel with them.)
Update: I found one more relevant quote, in Order of the Phoenix chapter 6.
Harry knew that Apparating was very difficult; it meant disappearing from one place and re-appearing almost instantly in another.
I take this as proof that Apparition is fast, but not as fast as the speed of light.
I'm sure it's slower than light-speed, or rather slower to activate and complete than light-speed. While Apparation may be instantaneous as far as the user is concerned, it has to be consciously activated, meaning you have to think about your destination before-hand and thus it can only act as fast as you can react.
One case in point is Dobby and Bellatrix Lestrange at Malfoy Manor. Dobby used Apparition to escape with several hostages but Bellatrix threw a knife at them as they were Apparating away. The knife stuck Dobby in the chest, causing his death upon his arrival.
Using this clip we can see that the knife was thrown before Dobby began Apparating and was transported along with Dobby and the hostages before they arrived:
This means that the knife reached Dobby before Apparation was completed so either the knife traveled instantaneously as well, or it was thrown at a speed faster than Dobby could Apparate. There's no evidence that Bellatrix used magic on the knife to make it travel faster, and we can clearly see it moving in flight. Thus, it can't be moving instantaneously, and thus neither can Apparation. On average, thrown knives travel between 26 and 30 mph, or 42-48 kph, and as there's no evidence that Bellatrix has superhuman strength or speed, this seems like a good speed range for the knife to travel at.
If Apparation was instantaneous then the knife would not have killed Dobby. Thus, it's probably safe to conclude that Apparation requires at least a second or two to be completed, as human reaction speed averages around 200 microseconds.
Rowling seems to indicate that when someone transports himself through apparition, he disappears from one place and appears at another instantly no matter what the distance. Afterall, where have fantasy and physics got in each other's way?
